SOIL MICROBIOLOGY Do Multi-year Applications of Bacillus thuringiensis subsp. israelensis for Control of Mosquito Larvae Affect the Abundance of B. cereus Group Populations in Riparian Wetland Soils? Salome Schneider 1,2 & Tania Tajrin 1 & Jan O. Lundström 3,4 & Niels B. Hendriksen 5 & Petter Melin 6 & Ingvar Sundh 1,7 Received: 20 January 2017 /Accepted: 30 May 2017 # Springer Science+Business Media New York 2017 Abstract Bacillus thuringiensis subsp. israelensis (Bti) is a soil-borne bacterium affiliated to the Bacillus cereus group (Bcg) and has been used in biocontrol products against nematoceran larvae for several decades. However, knowledge is limited on whether long-term Bti application can affect the structure of indigenous communities of Bcg and the overall abundance of Bti. Using species- and group-specific quantita- tive PCR assays, we measured the Bcg- and Bti-abundances in riparian wetlands in the River Dalälven floodplains of central Sweden. On five occasions during one vegetative season, soil samples were collected in alder swamps and wet meadows which had been treated with Bti for mosquito larvae control during the preceding 11 years, as well as in untreated control sites and well-drained forests in the same area. The average abundance of Bcg in alder swamps was around three times higher than in wet meadows. Across all sites and habitats, the Bti treatments had no effect on the Bcg-abundance, whereas the Bti-abundance was significantly higher in the treated than in the control sites. However, for individual sampling sites, abundances of Bti and Bcg were not correlated with the num- ber of Bti applications, indicating that added Bti possibly in- fluenced the total population of Bti in the short term but had only a limited effect in the longer term. The findings of this study increase the understanding of the ecology of Bti appli- cations for mosquito control, which can facilitate environmen- tal risk assessment in connection with approval of microbio- logical control agents. Keywords Bacteria . Biological control . Mosquito larvae . qPCR . Long-term effect . Sweden Introduction The soil-borne bacterium Bacillus thuringiensis (Bt) is a globally distributed insect pathogen. It belongs to the Bacillus cereus group (Bcg) which also includes B. cereus, B. anthracis, and the genetically more distant Bacillus mycoides, Bacillus pseudomycoides, and Bacillus weihenstephanensis [1]. Bt is most closely related to B. cereus [2], which is known as a human pathogen, causing diarrheal and emetic diseases [3]. Based on their high similarity, it is likely that Bt and B. cereus have a common ancestor [4]. Both species occur ubiquitously in the environment, and Hendriksen [2] argued that the ecology of Bt should be considered in association with that of B. cereus. Bt is the most widely used biological control agent and one of the most successful microbiological pest control systems [5]. More specifically, B. thuringiensis subsp. israelensis (Bti) Electronic supplementary material The online version of this article (doi:10.1007/s00248-017-1004-0) contains supplementary material, which is available to authorized users. * Salome Schneider salome.schneider@wsl.ch 1 Department of Microbiology, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden 2 Biodiversity and Conservation Biology, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland 3 Department of Medical Biochemistry and Microbiology (IMBIM), Uppsala University, Uppsala, Sweden 4 Swedish Biological Mosquito Control Project, Nedre Dalälvens Utvecklings AB, Gysinge, Sweden 5 Department of Environmental Science, Aarhus University, Roskilde, Denmark 6 Swedish Chemicals Agency, Sundbyberg, Sweden 7 Department of Molecular Sciences, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden Microb Ecol DOI 10.1007/s00248-017-1004-0